This invention relates to vinyl polymers that contain, as pendant units, organic moieties that possess high nonlinear optical activity.
Nonlinear optical activities generally result from interaction of materials with light, and are described in terms of second order nonlinearity, third order nonlinearity, and so on. An introduction to the theory and practical applications of nonlinearity, especially of organic materials, is provided by Nonlinear Optical Properties of Organic Molecules and Crystals, Volumes. 1 and 2, edited by D. S. Chemla and J. Zyss, Academic Press, 1987.
It is known that organic small molecules and polymeric materials with large delocalized .pi.-electron systems can exhibit nonlinear optical response, which in many cases is a much larger response than that exhibited by inorganic materials. Examples of such organic small molecules include 2-methyl-4-nitroaniline. Examples of such polymers are described in Nonlinear Optical Properties of Organic and Polymeric Materials, ed. D. J. Williams, ACS Symposium Series No. 233, American Chemical Society, Washington, D.C., 1983. Such materials generally contain in their nonlinear molecular units electron donor groups and acceptor groups linked by a conjugated .pi.-electron unit. This structural pattern gives rise to delocalization of the .pi.-electrons. The delocalized .pi.-electrons are believed to give rise to nonlinear effects when the material interacts with high intensity laser radiation. These effects are manifested as generation of different orders of light frequencies called harmonic frequencies.
While a nonlinear molecule can theoretically generate different orders of harmonic frequencies when it interacts with light, it is generally believed that in order to generate the even numbered harmonic frequencies such as second order, fourth order, and the like, the molecule must possess a "non-centrosymmetric" structure. The non-centrosymmetric structure may be inherent in the molecule or induced externally. A theoretical explanation of non-centrosymmetry and its relationship to harmonic generation can be found in Nonlinear Optical Properties of Organic and Polymeric Materials, referred to above.
In addition to the possibility of electronic interactions with light, organic and polymeric materials can be modified structurally to suitably optimize properties such as mechanical stability, thermooxidative stability, and laser damage threshold. Laser damage threshold is an expression of the ability of a material to withstand high intensity laser radiation. The utility of a nonlinear optical material frequently is in a device where the material is subjected to high intensity laser radiation. Unless the material is capable of withstanding such radiation, the device may fail in its intended function.
Furthermore, several organic polymers can be cast as thin films by techniques well known in the art. Thin films have the advantage of better utility than single crystals in device fabrication. Inorganic materials generally are single crystals.
Thin films of organic or polymeric materials with large second order nonlinearities in combination with silicon-based electronic circuitry have potential utilities in devices for laser modulation and deflection, information control in optical circuitry and the like. Novel processes occurring through third order nonlinearity such as degenerate four-wave mixing, whereby real-time processing of optical fields occurs, have potential utility in devices that have applications in such diverse fields as optical communications and integrated circuitry. Devices based on optical nonlinearity of materials are described in, for example, U.S. Pat. Nos. 3,234,475; 3,395,329; 3,694,055; 4,428,873; 4,515,429; 4,583,818; and by P. W. Smith et al in Applied Physics Letters, 30(6),280 (1977). Devices based on organic materials with conjugated electron systems are described, for example, in U.S. Pat. No. 4,865,406.
Nonlinear optical materials can be used either as small molecules in a guest-host combination or, more preferably, as a covalently linked part of organic polymers. Guest-host combinations are physical mixtures of a nonlinear small molecule and a film-forming polymer. Such mixtures have a number of disadvantages including insufficient loading of the nonlinear material, and possible phase separations. In contrast to the guest-host combination, polymer systems where the nonlinear optical moiety is covalently linked to the polymer chain avoid such disadvantages, and are generally referred to as nonlinear optical polymers.
Even though the individual nonlinear optical moieties in a polymer may possess inherently high activity, the overall activity in the polymer may be enhanced or reduced by orientation of the dipoles in the individual moieties. Thus, if the dipoles are oriented parallel to each other, the overall activity may be enhanced. If the dipoles are oriented opposite to each other, the overall activity may be substantially reduced or even zero. In order to enhance the overall activity of a polymer, the dipoles are typically oriented after the polymer is cast as a film. Several techniques, electrical, magnetic as well as mechanical, are available for such orientation, and are described in U.S. Pat. No. 4,913,844.
A well known and frequently used technique is "poling". During poling, the film is generally heated to a higher than ambient temperature, typically near the glass transition temperature (T.sub.g), and oriented in an applied electrical field; this orientation is then "frozen" in the polymer during a typical cooling process. Usually, better orientation is achieved by using higher electrical fields. However, the capacity to withstand high electrical field strengths differs among polymer films. Nonlinear optical polymers that can withstand high electric fields are preferred by those skilled in the art due to the possibility of achieving higher orientation of the dipoles.
Nonlinear moieties can be covalently linked to a polymer in either of two ways. They may exist as part of the main chain of the polymer or as pendant side groups. For example, EP 89402476.9 discloses main chain polymers such as polyurethanes or polyesters formed from difunctional nonlinear optical materials. A typical example disclosed has recurring units shown in Formula I: ##STR2## In Formula I, the nonlinear optical moiety (indicated by the grouping starting from N. and terminating at the two cyano groups) contains the nitrogen of the amine functionality as the electron donor, and the cyano groups as the electron acceptor, linked via a conjugated electron system. The electron donor nitrogen is part of the main chain polymer backbone.
Examples of polymers that contain nonlinear optical moieties as pendant side groups are described in U.S. Pat. Nos. 4,779,961; 4,801,670; 4,808,332; 4,865,430 and 4,913,844. The polymers disclosed by, for example, U.S. Pat. No. 4,865,430 include materials of Formula II: ##STR3## where m and m.sup.1 are integers which total at least 10, and the m monomer comprises between about 10-90 mole percent of the total m+m .sup.1 monomer units; R is hydrogen or a C1-C4 alkyl, C6-C10 aryl, halo or haloalkyl substituent; n is an integer between land about 12; R.sup.1 is a C1-C6 alkyl substituent; R.sub.2 is hydrogen or a C1-C4 alkyl substituent; and Z is a nitro or cyano substituent. The material of Example I in the same patent is shown in Formula III: ##STR4## In Formula III, the nitrogen donor atom and the nitro acceptor group are linked via a stilbene unit, and the nonlinear optical moiety is attached to the polymer backbone as a side chain. This material of Formula III is a copolymer, formed from two comonomers. In the case of copolymers, the comonomer or comonomers chosen may also carry nonlinear optical side chains. Additionally, comonomers can be suitably chosen to enhance the quality and transparency of the films obtained from the copolymer. The choice of a wide variety of comonomers available renders fine tuning of polymer properties readily achievable.
While such polymers exhibit good nonlinear optical activity, increasing sophistication of devices demands higher levels of such activity in polymers. Thus, there is a continuing interest in the preparation of novel polymers and copolymers containing nonlinear optical moieties with high activity. There is also an increased effort to develop novel nonlinear optical devices based on such polymers.
Accordingly, it is an object of this invention to provide novel vinyl polymers with pendant side chains which exhibit high nonlinear optical response.
It is yet another object of this invention to provide novel side chain vinyl polymers which can be poled at high field strengths.
It is a further object of this invention to provide nonlinear optical media incorporating a transparent nonlinear optical component which comprises such vinyl polymers.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.